1. Field of the Invention
The present invention relates to a current-controlled semiconductor device and a control unit using the same, and particularly to a current-controlled semiconductor device suitable for incorporating a current detection circuit into an IC chip and a control unit using the same.
2. Description of the Related Art
As various targets to be controlled become electrically controlled, electromotive actuators such as a motor, a solenoid, etc. are being widely utilized to convert an electric signal into mechanical motion or hydraulic pressure. High-accuracy current control is essential to greater sophistication of these electromotive actuators. In recent years, it has become a common practice to use digital feedback control for the purpose of the high-accuracy current control.
The digital feedback control on the current needs to acquire a digital value of a current value Iout to be controlled. To this end, an output Vout of a current-voltage conversion circuit is digitally converted by an AD converter to obtain a digital value Voutd (=Vout/Vref) corresponding to a value relative to a reference voltage Vref of the AD converter. Next, a correction corresponding to the input/output characteristics of a current detection circuit including the current-voltage conversion circuit and the AD converter is performed on the output Voutd of the AD converter to obtain a digital value of a current value Ioutd.
Various configurations are considered for the current detection circuit, but the input/output characteristics of the current detection circuit are desirably linear in terms of simplification of a control algorithm. In this case, the digital current value Ioutd is determined from the following equation (1) using a gain a and an offset b:Ioutd=a·Voutd+b  (1)
How the gain a and the offset b should be made coincident with the actual characteristics of the current detection circuit with satisfactory accuracy where the current value is measured in accordance with the equation (1), is important in improving the accuracy of current measurements.
There is also a growing need for a reduction in the size and cost of a control unit in addition to the high-accuracy current control. Provision is made for it by incorporating the current detection circuit into an IC chip. There has been known one in which a current detecting resistor is incorporated into the IC chip, for the purpose of incorporating the current detection circuit into the IC chip here (refer to, for example, JP-2003-203805-A and JP-2006-165100-A).
Incorporating the current detecting resistor into the IC chip as described in JP-2003-203805-A and JP-2006-165100-A enables a reduction in the number of external parts for current detection and is a method excellent in the size and cost reduction of the control unit.
The value of the resistor formed within the IC chip, however, fluctuates a few tens of percent with the temperature. This appears as a fluctuation in the gain a of the equation (1) as it is. A fluctuation in the reference voltage Vref of the AD converter used for the digital conversion of the detected current value also leads to a few percent fluctuation in the gain a. Further, the offset b of the equation (1) also fluctuates a few percent due to an input offset of an operational amplifier used in the current detection circuit.
A problem arises in that when the current detection circuit is incorporated into the IC chip, the gain a and the offset b in the equation (1) greatly fluctuate as compared with design values, thus causing an increase in current detection error.